The Boss choose one player to perform an ability called "Purge". The boss visually charges his power and after a brief pause, attacks the targeted player. The attack does a base amount of damage and can be split up and mitigated if the other players stand beside the targeted player while he is hit. Without this mitigation, the attacked player would surely die. If the damage is split up amongst enough players, there is severely less risk. The boss also covers large portions of ground in fire with an ability called "Wildfire", dealing damage over time. He also does other large attacks that damages all players within a certain radius of it's target location called "Lava Flow". This attack picks a player and attacks in his direction rather than at the player directly, giving him a short time to get out of the way. If all of the players are standing close together, they all could suffer a lot of damage. There is no mitigation here to reduce damage. The Boss also has one last ability called "Inferno Aurora". The closer the players are standing to the boss, the more damage they take from fire damage. This makes Melee classes a target of priority for Healing classes when trying to deal damage to the boss.
This fight ensures that players remain vigilant of the boss' attacks and forces them to cooperate frequently. The constant movement makes powerful charged spells difficult and keeps the players jumping around like their pants are on fire. I consider this boss fight much more dangerous than the Lighting Bosses because one lapse in concentration and cooperation could very quickly lead to someone's death. This boss fight does not f*** around.
If my idea's seem very well thought out, original, and organized, it's because they're a general description of Boss mechanics that are planned to be included in my own MORPG. I've given them all quite a bit of thought to make sure that teamwork is very necessary for beating the boss. Even though I give you the idea's to use, I still plan on using them myself. It's your choice to add what you need to make it fit the theme of your project, I'm just giving you a main mechanic for a fight. In this case, you may also want to rename specific abilities and status effects.
Keep in mind that unless you seriously scale down this boss' abilities, it should only be left as a high level boss fight for experienced players. I built this fight to literally tear out each player's a-hole through their nostril.
1) A skill that attacks everything on the screen by spawning fire/spikes in a box(simplest implamentation of this) around the player, then spreading into a bigger box until out of view, meanwhile the player is frozen, leaving him open to anyone who can survive/get past the skill(balancing it out imo).
2) A skill that sends a shuriken(or series of them(three+ in same icon)) from mob to mob in the view(been implamented in games before). This would cause very little amounts of damage, which would balance it.
3) A skill that enhances movement speed(pretty generic).
4) A skill that sacrafices half of the users maxium health, which can result in death, but deals twice the amount of damage to the opponite(I havn't seen it done before, but it's simple enough to have been done).
Items:
1) I'll list a bunch of generic items in this one by their selves. Special Kunai that increases attack speed. Boots that increase movement speed. Explosives that can be attached to weapons to deal splash damage.
2) 'Boomerang' throwable weapons that will attack in a line, then turn around, dealing damage to anything in it's path. This would cleary freeze the user, leaving them open aswell.
Objects:
I only got one idea; pressure plates to use in puzzles, these will 'press' whenever a object/mob enters the weight plate. This could be used with the Item #2 or the skill #1, where certain pressure plates are needed to be pressed down.
Of course, offering movable objects in the area to move onto pressure plates would also be available for a more long-gated process.
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Forgive me for not reading other replies, if I offered any of the same ideas as anyone else.